A number of chemical vapor deposition (CVD) techniques including hot filament CVD (HFCVD), RF plasma assisted CVD, microwave plasma assisted CVD, DC plasma assisted CVD and laser assisted CVD methods have been used to deposit thin (1-10 .mu.m), adherent and coherent PCD films on a variety of substrates. However, these methods have not been successful in depositing thick (.gtoreq.12 .mu.m) PCD films adherently and coherently on metallic and ceramic substrates. Furthermore, the films deposited by these techniques have been found to have poor electrical properties, making them unsuitable for the electronics industry.
The electrical properties of PCD films can be greatly improved by depositing them with enhanced crystal orientation in the (220) and (400) planes, as disclosed in a commonly assigned copending patent application, U.S. patent Ser. No. 497,161, filed 20 Mar. 1990, now abandoned. These PCD films have been successfully deposited at low as well as high rates on metallic substrates such as molybdenum and ceramics such as silicon with good adhesion. The adhesion has been shown to be extremely good as long as the film thickness is limited to 10 .mu.m. Although it is possible to deposit thicker films (&gt;10 .mu.m) at high rates both on molybdenum and silicon, their adhesion to these substrates has been noted to be poor. The PCD films on molybdenum have been found to simply flake off during cooling of the coated specimens from the deposition temperature to room temperature. Likewise, the films on silicon have been noted to be under high stress, causing the coated silicon to disintegrate into pieces. The disadvantages of such thick PCD films are set forth more fully below.
Several attempts have been made by researchers to deposit thick PCD films on metallic and ceramic substrates with limited success. The differences between the coefficients of thermal expansion of diamond and metals cause the thick films to separate from metallic substrates as the coated substrates cool from deposition temperature to room temperature, as reported by Peter Taborek in a recent paper entitled, "Optical Properties of Microcrystalline CVD Diamond," published in SPIE, Vol. 1112, Window and Dome Technologies and Materials, 205-209 (1989).
The thick films have, however, been reported to adhere well to silicon substrate, but they have been found to be under high stress (apparent from the resulting curvature of the substrate). In some cases the stress is great enough to cause the sample to disintegrate into pieces, as reported by D. Morrison and J. A. Savage in a paper entitled, "Optical Characteristics of Diamond Grown by Plasma Assisted Chemical Vapor Deposition," published in SPIE, Vol. 1112, Window and Dome Technologies and Materials, 186-191 (1989). Therefore, there is a need to develop technology to deposit thick PCD film adherently and coherently on metallic and ceramic substrates.
Japanese Kokai Patent No. Sho 63(1988)-307196, published 14 Dec. 1988, discloses a microwave plasma assisted CVD method of manufacturing multilayered PCD film preferentially oriented in the (220) crystalline direction. In this patent application, the diamond deposition conditions such as the concentration of methane in hydrogen are changed continuously or discontinuously to deposit distinct diamond layers with different properties. For example, the first layer of the microcrystal diamond film with 0.1 .mu.m thickness is formed using high concentrations of methane in hydrogen (such as 2%). The second layer is deposited on the first layer with good crystallinity using low concentrations of methane in hydrogen (such as 0.3%). This application does not disclose a method of depositing thick, uniform, adherent and coherent PCD film on a substrate.
U.S. Pat. No. 4,816,286 discloses an HFCVD method for depositing PCD film to a thickness as high as 28 .mu.m on various substrates at deposition rates of about 3 .mu.m per hour and higher; see Examples 1-8 starting at column 5, line 48 through Table 1 bridging columns 7 and 8. It has been found that at this rate of deposition the adhesion of PCD films to the substrates is poor.
Thin (10 .mu.m) PCD films are suitable for many applications including low-power, direct-current, or low-frequency devices for dissipating heat from the devices as well as for isolating the devices from the base materials. They are, however, not suitable for high-frequency and/or high-power devices with large areas because of their high capacitance. The desired value of capacitance for these devices is .ltoreq.3 pF, requiring the use of thick PCD films for these applications. The thickness of a PCD film required for a particular application depends largely upon the device area and can be calculated by the following expression: ##EQU1## where: C=capacitance of PCD film in pF
K=dielectric constant of PCD film (assumed to be 5.5 for diamond) PA1 A=device or chip area (cm.sup.2) PA1 t=PCD film thickness (cm) PA1 E.sub.o =free-space permittivity (8.85.times.10.sup.-2 pF/cm)
A relationship between PCD film thickness and device or chip area can thus be established by plugging in the values of E.sub.o, K and desired capacitance in the above equation. The relationship between film thickness and device area can therefore be represented by the following expression: EQU t.gtoreq.0.162 A
This expression can be used to calculate the thickness of PCD film required for devices having different areas, and the calculated values are summarized below. ##STR1##
These values indicate that .about.10 .mu.m thick PCD films will be suitable only for devices with area &lt;10.times.10.sup.-3 cm.sup.2. The devices commonly used by the electronics industry have areas .gtoreq.10.times.10.sup.-3 cm.sup.2, suggesting that the film thickness has to be .gtoreq.16 .mu.m to meet capacitance requirement. Therefore, there is a need for depositing thick PCD films on metallic and ceramic substrates with good adhesion and electrical properties. Further, the surface finish of polycrystalline diamond films can also be enhanced over that of prior art PCD films by depositing diamond crystals with enhanced orientation in at least two directions. This is an important feature in regard to mounting the device on the PCD film.